Fluorescence in situ hybridization (FISH) is routinely used for gene mapping, detecting pathogenic DNA, and diagnosing cancers and genetic diseases resulting from chromosomal aberrations such as translocations and gene amplifications. As a result of increasing use of reproductive assistance methods, such as in vitro fertilization, a relatively new application of FISH has recently emerged. For use in Preimplantation Genetic Diagnosis, or PGD, FISH has become a critical technique for determining which fertilized embryos are the most suitable for implantation. Due to the need to implant fertilized eggs at a specific time in development, FISH results must be sensitive and rapidly available requiring new reagents and procedures. Using a novel probe labeling method, this SBIR aims to develop a rapid and sensitive FISH test for simultaneously detecting human X and Y chromosomes for PGD and stem cell research. The ultimate goal of this SBIR project is to develop a full set of chromosome specific FISH probes for PGD.